1. Field of the Invention
The present invention relates to an apparatus for monitoring a boundary; more specifically for detecting whether or not a ball or foot has hit a line on a game court.
2. Description of the Prior Art
In tennis, like many other sporting events having boundaries of play, judgement calls must be made continually as to whether events have taken place "in" or "out" of the boundary. In which involves ball velocities of 100 miles per hour or greater, such judgement calls are extremely difficult because of the inherent inability of the human eye to follow an object moving at such a speed, and relate the relative position of impact on or over a line. Such calls are further complicated by the fact that a tennis ball often slides or rolls several inches on the ground before rebounding into the air adjacent the court surface.
Prior to this time, a number of devices have been proposed which replace human judgement with a mechanical or optical sensor. The shortcomings of these devices, however, have prevented the widespread acceptance of any of these devices. Many of the previous devices require modification of the ball to plot its position relative to electrical conductors provided beneath the court and or its boundaries. This may be unacceptable in regulation play, and greatly increases the cost of tennis balls and of modifying existing courts.
A second type of device is one which is laid on the playing surface and senses the impact of the ball or player on the critical boundary. Prior to the present invention, these sensors were made of materials not well suited to withstand the strain of multiple impacts over a long period of time. For example, in U.S. Pat. No. 4,365,805, a pressure switch, known as a "membrane" switch is comprised of foil conductors separated by a dielectric with holes. The weight of the impact presses the foil together within holes formed in the dielectric material. The switches depend on plastic deformation of the conductors. Repeated deformations of this type, particularly under heavy pressure, tend to stretch the conductors, which over time may cause an electrical failure. Typically, the onset of failure is indicated by intermittent or multiple switching contacts per actual impact on the sensing device. This unreliability over time is unacceptable.
Finally, a serious problem is "shadowing." This is the masking of one impact by another. For example, if a foot is already on the line when the ball hits, the sensor must be able to still detect the second impact, even if the two events are quite close together or simultaneous.
Prior to the present invention, there has been no comprehensive solution to these problems.